I am insulating an outbuilding I have. I know that traditionally the vapor barrior goes on the "warm in the winter" side and that is how I am handling all the exterior walls.

However, I have 2 twists on this I need help with:

1. The outbuilding will be divided into 2 halves on the main level and separated by a wall. One side will be a garage and the other a workshop. Both will be heated, but to different temperatures. The garage will be kept at just above freezing (35-40 F or 2-4 C) while the shop will generally be kept around 55 F/12 C when I am not using it and around 65-70 F/18-21F when I am in there or need the extra heat. So which side does the vapor barrier go on?

2. There is also a loft that I want to turn into a "guest house". It is above both the garage and the shop. Most of the time it will be keep it just above freezing so the pipes don't freeze, however when someone is staying there it will be whatever temperature is comfortable for them. Which side does the vapor barrior go on in this case?

It's not really which side is warmer than the other because when a wall has fairly cold and fairly dry air on both sides of it, then there isn't enough moisture in the air on either side to do any harm if it gets into the insulation in between. It's only when you have warm enough and humid enough air on one side of a wall and cool enough temperatures on the other side that would condense that humidity to condense into water and/or ice as it passes through the insulation that you need a vapour barrier to prevent that from happening.

In the first case, it matters little because you generally won't have as high humidity in the shop where the only source of moisture is your breathing during the times you're in there. (in a house you have cooking, cleaning, showering, bathing, plants, pets, etc. with 24/7 warm temperatures so you have both warm AND humid air all the time) However, if you had to choose, the vapour barrier should go on the shop side. The reason why is that the purpose of a vapour barrier is to prevent humid air from the warm side of the insulation from getting into the insulation and condensing to form water as it moves to the cold side of the insulation. That water will get the insulation wet and thereby greatly reduce it's ability to insulate. Also, because insulation works by keeping air stagnant, wet insulation takes forever to dry out, and that can lead to wood rot and mold growing inside the insulated walls. Still, if this is just a workshop, you probably won't get enough humidity in there to cause a problem even without a vapour barrier between the shop and garage.

As for the ceiling, you can have moisture flowing in either direction in that case. Warm air from the shop rises, and so you need to prevent that warmest and moistest air just below the ceiling from getting into the insulation between the ceiling joists. But, when the shop isn't being used but the loft is, then you need to prevent the warm moist air in the loft from getting into the insulation in the loftfloor/shopceiling. In a case like that, the WORST thing to do is put vapour barrier on both sides of the insulation.

What you should do is avoid the problem entirely by switching to an EXTRUDED polystyrene insulation like Roofmate. Extruded polystyrene insulation is "closed cell foam", which means that they use less blowing gas when making the foam so that the tiny air bubbles inside the foam aren't interconnected. That effectively makes the insulation impermeable to air for all intents and purposes, and so it doesn't need a vapor barrier. You simply stick thick slabs of roofmate into the spaces between your ceiling joists and caulk on both sides with expanding foam, which will also hold the insulation in place as well as plug any gaps to prevent air leakage past it.

Now you also know how to design the walls of a duplex, triplex or 4plex, where any or all of the living units can be occupied or empty in winter, depending on the rental situation.

Typically EXTRUDED polystyrene insulation will be blue or pink in colour, whereas EXPANDED polystyrene insulation will be white in colour and will be the "foam bead" insulation that you see used for making everything from packing materials to cheap camping coolers. EXPANDED polystyrene can have interconnected air bubbles and isn't considered "impermeable" to air, even though it's permeability to air is generally very low. It's that interconnecting of air bubbles in expanded polystyrene insulation that can actually cause it to become water logged (full of water) when exposed to water. This is a well known problem in some refrigerators where the expanded polystyrene parts need to be replaced when they become water logged. The extruding process can't be used to make foam parts of variable thickness or complex shape, so extruded polystyrene foam will always be of uniform thickness and uniform shape. Extruded foam insulation has a little higher R value than the expanded stuff, but it's not much different. It's only that the expanded foam can have some permeability to air and can actually flood with water that makes it a "second choice" for insulating, with extruded polystyrene insulation always the first choice (if you can afford it's higher price).

You could address your concerns by using expanded foam insulation in the wall between the shop and garage, too.

Turning to the garage, I'd probably insulate the exterior garage walls to hoard as much heat as possible from parking 2500 pounds of Detroit steel in there every night. Insulating the garage walls will help keep he heat in so that you have less heat loss from the shop or loft when a recently driven car is in the garage, AND it might be enough to keep the garage a few degrees warmer when you go out to start your car every winter morning to go to work. Vapour barrier over the insulation in the garage exterior walls isn't really necessary, but the stuff is cheap and installs quickly, so for the $10 to $20 it's gonna cost, I'd probably put it up before drywalling.

If you live in Canada, you should buy "polyethylene sheeting" instead of "vapour barrier". That's cuz in order for polyethylene sheeting to be called "vapour barrier" in Canada, it has to have a minimum thickness of 6 mil (6 thousandths of an inch). Polyethylene sheeting, on the other hand, comes in various average thicknesses and doesn't need to be any minimum thickness to be called "polyethylene sheeting". So, because the manufacturer has to guarantee that it's at least 6 mil in thickness to call it "vapour barrier" and sell it as such, they charge a lot more for "vapour barrier" than they do for "polyethylene sheeting". Considering you don't even need the stuff for your garage exterior walls, it would be smartEST to use polyethylene sheeting instead of vapour barrier on the garage exterior walls.